1. Field of Invention
This invention relates to a computer network line protocol, and more particularly to a line protocol for providing the asynchronous exchange of digital signal information between electronic processing units of a supervisory control system.
2. Description of the Prior Art
Line protocols for controlling intracomputer communications are well known in the art and are required to organize the transmission of data from one computer to another in a manner which assures correct sequencing and data integrity. The line protocols presently available each have particular characteristics and advantages related to the type of interface over which data is transferred, i.e. full or half-duplex, synchronous or asynchronous, serial or parallel, etc. An excellent primer on the subject may be found in a book by E. B. Stelmach, entitled "Introduction to Mini-Computer Networks", Copyright 1974 by Digital Equipment Corporation, Chapter 5.
All of the known line protocols have a common characteristic, each requires a significant number of dedicated, overhead signal bits within the transferred data message format, i.e., signal bits that are required for use other than for the transfer of the data information. These bits are required to ensure the accuracy of the data transmitted in the information field of the message block. Where a large number of data bits are transmitted in each message block, such as in batch transfers using a synchronous data link control (SDLC) protocol, the header field positions (start flag, address field and control field) and trailer field (frame checking and stop flag) may each comprise twenty-four bits for a total of forty-eight bits in each frame. These dedicated signal bits are an overhead cost in the transmission of data under the particular line protocol format since they require some portion of the line utilization time period which may otherwise be used for the transfer of data. This results in some degree of loss in throughput efficiency of the line. In volume data transfers, i.e. "batch transfers" in which the typical information field is on the order of two thousand bits, the overhead bits required by the protocol represent a small percentage of the total bits in each message. In contrast, intercommunication protocol between computers in a supervisory type control system, such as that disclosed in the hereinbefore referenced copending applications in which a central processing unit (CPU) controls and supervises the operation of some number of remote processing units (RPUs), the CPU is constantly corresponding with all RPUs at comparatively high band rates, with the communication to each individual RPU comprising relatively short message lengths interspersed with comparatively long pause intervals. There is no intercommunication between RPUs, but only between each RPU and the CPU, such that the required protocol format is essentially a link control format.
Due to the short messsage lengths involved in the information transfer, on the order of one hundred bits per message unit, the line protocol must have a low bit overhead to permit a reasonable throughput efficiency while still allowing for the highest accuracy possible, i.e. intercommunication integrity, since in such a control system erratic or faulty transmissions could result in catastrophic failure of the control item at the remote sites. Similarly, such a control system does not permit for synchronous data transmission since the nature of the installation is that of a network of hard wired remote units where the cost for installation and maintenance of separate clock lines over the long distances between units is impractical. Therefore, the protocol must ensure low overhead to allow for the highest throughput rates, error control to ensure the highest accuracy to avoid any catastrophic failures, and provide for asynchronous exchange of signal information between processing units.